Blood Pressure Monitor

Background

Blood pressure is the pressure that the blood exerts against the walls of
the arteries as it passes through them. Pulse refers to the periodic
ejection of blood from the heart's left ventricle into the aorta.
The left ventricle, or chamber, receives blood from the left atrium,
another of the heart's chambers. By contracting, the left ventricle
drives the blood into the aorta, a central artery through which blood is
relayed into the arteries of all limbs and organs except the lungs. Pulse,
transmitted though the arteries as a repeated pressure wave, is the
mechanism that moves blood through the body.

The high and low points of this pressure wave are measured with the
sphygmomanometer,
or blood pressure monitor, and are expressed numerically in millimeters
of mercury. The higher number,
systolic pressure,
measures the maximum pressure exerted on arteries and the heart muscle;
the lower figure,
diastolic pressure,
measures the minimum pressure exerted. The reading of the two
measurements indicates how hard the human system is working. All
physicians consider a patient's blood pressure when determining
general health or diagnosing disease.

The blood pressure monitor is used in conjunction with a stethoscope.
After fastening the constricting band, or cuff, around one of the
patient's arms above the elbow, the clinician inflates the cuff by
pumping air into it with a rubber squeeze bulb until the mercury column or
the needle of the gauge (also known as an
aneroid dial)
stops moving, usually at a point between 150 and 200 millimeters of
mercury. The stethoscope is then placed over the brachial artery, on the
inside of the arm at the elbow, while air is slowly released from the
system via a small valve attached to the bulb. The technician watches
carefully as the air escapes and the pressure indicator correspondingly
declines. The point on the gauge at which the pulse can first be heard
through the stethoscope indicates systolic pressure, and the
gauge's reading when the sound disappears indicates diastolic
pressure. Normal pressures vary with the individual, but systolic pressure
typically ranges between 110 and 140, while diastolic runs from 65 to 80.
Pressure above normal levels predisposes the patient to such health
problems as heart disease, stroke, and kidney failure.

Many early attempts at measuring blood pressure involved attaching an
instrument directly to one of the patient's arteries, a painful and
dangerous practice. The first sphygmomanometer to use an inflated armband
was developed in 1876 by Samuel Siegfried von Basch. Twenty years later,
the Italian physician Scipione Riva-Rocci developed a more accurate device
that soon replaced von Basch's instrument. Riva-Rocci's
design was much like today's monitor, but its operating procedure
allowed for measuring blood pressure only while the heart was contracted.
In 1905, the measurement procedure was further refined by Nikolai
Korotkoff, who added the use of a stethoscope to detect pulse rate,
thereby enabling doctors to measure blood pressure while the heart was
relaxed as well. Korotkoff suspected that both pressure readings were
important, and today we realize that certain indications, such as a rise
in the systolic with a stable or falling diastolic pressure, may suggest
brain damage.

The neoprene bulb is commonly made using vacuum-assisted injection
molding. In this process, molten neoprene is injected into a mold of
the proper shape. The mold is equipped with tiny holes, through which
the air in the chamber is drawn out just before the neoprene enters.
The resulting vacuum causes the neoprene to flow into the cavity
evenly. Within a few seconds after injection, the neoprene cools and
hardens and can be removed.
The pressure gauge contains two phosphor-bronze disks soldered
together. Some blood pressure monitors utilize either a mercury
manometer or an electronic display.

Design

All blood pressure monitors feature an air pump device equipped with a
control valve, a means of indicating pressure, a constricting band to be
attached to the patient, and the various connecting hoses that operate the
system. Although three distinct types of blood pressure monitors exist,
they differ basically in their means of registering pressure: one type
uses a pressure gauge or dial; another type uses a mercury manometer (a
manometer is an instrument that measures the pressure of liquids and
gases); and the third uses an electronic or digital display. Despite the
availability of electronic display sphygmomanometers, instruments that use
a manometer or a dial are still more popular because they are easier to
service as well as accurate, durable, and inexpensive. This article will
focus on the dial or pressure gauge type.

A typical blood pressure monitor features a neoprene or rubber pump bulb
that a medical technician squeezes to build air pressure in the system.
Increasing air pressure inflates the constricting band and provides a
pressure signal to the manometer or indicating gauge. The process is
controlled by a valve, which has a hose fitting to attach the tube leading
to the constricting band and gauge. Integral to the valve is a one-way
flow device that operates only when the valve is closed. It usually
consists of a small rubber disk or ball that is placed over the air
passage from the squeeze bulb opening and secured by a screw or clip. The
compressed air raises the ball slightly when the bulb is squeezed, sealing
the opening to the atmosphere and forcing air to enter the cuff. Upon
release of the bulb, the ball seals the opening between the bulb and the
hose, opening the former to the atmosphere and allowing it to refill with
air. This cycle is repeated until the correct starting pressure is
reached. The manual valve opens a bypass route to release the air while
readings are being taken.

The rubber hoses are made by continuous extrusion, in which molten
rubber is forced through a die block by a rotating screw device.
Within the block is a rod the same size as the inside of the tubing;
as the rubber flows around this rod and out of the die, it cools and
assumes the shape of the tubing. It is then cut to the proper length.

Raw Materials

The dial, or aneroid type, instrument is a mechanical pressure gauge that
has a pointer and dial calibrated in millimeters of mercury. The pressure
gauge consists of three basic groups of parts: a pressure element and
socket assembly; a movement and dial assembly; and a protective case and
lens assembly enclosing them. The pressure element comprises two
phosphor-bronze disks of approximately .010 inch (.025 centimeter) with a
formed lip on the outer edge. The movement is usually made of
polycarbonate and brass materials and contains a small gear train that
amplifies the short travel distance of the disks. The movement assembly
also supports the dial, which may be brass, aluminum, or plastic. The
output shaft of the movement is mounted with the aluminum pointer.

The squeeze bulb is usually rubber or neoprene, as are the connecting
hoses. The band, or cuff, is basically a fabric-covered neoprene bladder
with a hook and loop (Velcro) fastener. The bladder is enclosed in a nylon
or synthetic fiber fabric, which protects it from cuts during use by
on-scene rescue technicians and reduces patient discomfort. The band must
be very flexible and durable to accommodate the infinite differences in
patients and situations. The control valve can be made of polycarbonate,
brass,
stainless steel,
or combinations of these materials.

The Manufacturing
Process

Many manufacturers purchase the components of the blood pressure monitor
separately, then assemble and package the unit for sale. Each part has its
own manufacturing and assembly process.

The bulb

1 The bulb can be made using various processes, but it is most commonly
produced using vacuum-assisted injection molding. Compressed air is used
to blow the melted rubber or neoprene material into the cavity of a
two-piece metal die set featuring the negative image of the bulb (the
operation resembles blowing a gum bubble inside a bottle). The die also
contains small holes through which air is drawn out just before the
material is injected, helping it flow into the die cavity at a uniform
thickness. While these holes are large enough to allow air to escape,
they are too small to permit significant amounts of the rubber to seep
out. The remnant of the rubber material that is drawn into the holes
produces small protuberances that resemble the small
"whiskers" visible on a new tire. Within a few seconds of
its injection, the material has cooled so that the die may be opened,
revealing the finished bulb. After a minimal amount of hand work to
remove the whiskers, the bulb is ready to be attached to the other
components.

The valves

2 The valves are made by die casting, plastic injection molding, and
machining from bar material. They incorporate connection features that
allow the bulb and hose to be attached. Machined valves can be made on a
lathe controlled by a computer program that instructs it to turn the
shape, threads, and other features.

A finished blood pressure monitor. While monitors that use pressure
gauges for pressure display will continue to be popular because of
their portability, electronic displays will increase in use as new
power sources are developed and the design is made more rugged.
Mercury monitors will likely drop from favor because of the hazardous
effects of mercury.

The gauge

3 The gauge consists of further sub-assemblies, each of which contains
machined, molded, and stamped parts. The most important component of the
gauge is the pressure element. It is constructed by soldering two disks
together at the formed lip to construct a hollow wafer. The pressure
from the system is introduced into the wafer though a hole in the socket
connection, which in turn is connected to the squeeze bulb and cuff. As
the internal pressure increases, the wafer swells. It is this swelling
that is detected by the movement assembly, causing the pointer to rotate
about the dial. After assembly, the gauge must be calibrated. This is
accomplished by connecting it to a pressure source with a master gauge
of known accuracy. Slight adjustments are made in the movement linkage
until the pointer of the gauge reflects the correct pressure readings.

The cuff

4 The constricting cuff, or bladder, is made by heat sealing two rubber
sheets together to form a flexible band. A tubing fitting is
incorporated into this sealing process, providing a connection for the
air supply. A fabric covering is then sewn to the bladder by
conventional methods.

The hoses

5 The hoses are made by continuous extrusion, a process in which pellets
of rubber or similar material are heated to the melting point, at which
they become clay-like and viscous. Within the same machine, a rotating
screw device forces this molten material through a die block, which is
simply a hole in an aluminum block the same size as the outside of the
tubing. Secured within the block is a rod that is the same size as the
inside of the tubing and positioned in the center of the hole. As the
material flows around the rod and out the hole, it cools and assumes the
shape of the tubing. At this point, it is cut to length and coiled onto
spools for shipment to the assembly facility.

Assembling the components

6 At final assembly, hoses are used to connect the components discussed
above. The hoses are then checked for leaks and the calibration is
verified. This is a good example of JIT
(just-in-time)
material requirements planning and TQC
(total-quality-concept)
management. Missing any one of the components, the entire assembly is
useless. The plant must receive the parts and supplies in a timely
manner to assure delivery of the finished product to the customer. The
items must be of satisfactory quality, so they can be assembled
correctly and without compromising the design. Many companies today have
established quality management procedures. These procedures are simply
intensive studies of all aspects of manufacture to eliminate or reduce
the possibility of producing a defective part. It is not just making the
part, but also designing it, selecting the materials, choosing packaging
selection, and all other
aspects that determine the quality of the finished product.

The Future

Medical product manufacturers and their suppliers are prone to liability
suit due to failure (or perceived failure) of their products. A portion of
the cost of the instruments stems from the costs of insuring and defending
the company against these lawsuits. Many companies have discontinued
products because the liability risk is too much of a financial burden for
them. For example, the mercury type instrument will probably be
discontinued due to hazardous materials issues as discussed above. The
electronic versions will most likely increase as new power source designs
and improved ruggedness are achieved. Medical technicians and therapists
rely on measurements of blood pressure as a benchmark for health, and
consequently, some type of sphygmomanometer will always be used.

Where To Learn More

Books

Emergency Care and Transportation of the Sick and Injured.
The American Academy of Orthopedic Surgeons.

Sphygmomanometers: Electronic or Automated.
Association for the Advancement of Medical Instrumentation, 1987.

Travers, Bridget, ed.
World of Invention.
Gale Research, 1994.

Periodicals

Calem, Robert E. "Monitoring Blood Pressure Without Skipping a
Heartbeat,"
New York Times.
March 28,1993, p. F16 (N).

—
Douglas
E.
Betts

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